It is through an understanding of the programming of gene expression that we shall arrive at a conception of cell transformation as it is manifested both in cellular differentiation and in oncogenesis. An important conceptual aspect of cancer may thus not be realized except through an understanding of the genetic basis of cellular differentiation. The crucial experimental problem appears to be to discriminate between those events of macromolecular synthesis and interaction that have a bearing on the transmission and regulation of specific genetic information and those that are part of a widely encompassing program of genetic expression. The system we have chosen, to elucidate these processes, is the sea urchin embryo, whose development from the blastula to the late gastrula appears to involve a transition from a minimally differentiated state to an increasing degree of cellular specialization. We shall examine the expression in the embryo of what now appear to be three distinct gene classes, characterized by their transcribed mRNAs, the histone genes and two classes of non-histone genes. Our procedures will use mainly DNA-RNA hybridization, complementary DNA formed by reverse transcriptase and in vitro translating systems for mRNA templates. We shall also characterize three classes of heterogeneous nuclear RNA, which are demonstrable as having distinctly different properties. The three nuclear RNA classes and the mRNA of the three gene classes change in their relative synthesis during embryonic development. We shall attempt to decipher the significance and functions of these different RNA classes, especially with respect to embryonic development.